This invention relates to a portable device and method of measuring enthalpy or heat content of humid air streams.
Many industrial processes involve the flow of humid air to or from the pieces of equipment. Generally, such processes are involved in removal of water from a product (drying) or the re-entry of water to improve the physical properties of the product (conditioning). Considerable energy is transferred in the humid air streams of these processes.
When designing or modifying existing equipment, a knowledge of the heat entering the leaving the equipment is essential for efficient operation. The need for such information is more critical in processes where stream temperatures go above 200.degree. F. because of the potential for energy recovery from these streams for further use in a processing plant.
We know of no device which measures the energy content of humid air streams. Energy content of air streams are usually evaluated indirectly by heat and mass balances on a process from measurements of temperatures, input energy, and humidities.
In humid air streams above 200.degree. F., the amount of water vapor in the air can contribute significantly to the total heat content of the stream. Therefore, it is essential that humidity content be accurately measured. We previously developed a psychrometer for measuring the humidity of a gas flow up to 500.degree. F. dry-bulb and 210.degree. F. wet-bulb temperature. This device has a double-wick wet-bulb sensor, which enables accurate and rapid determination of humidity at temperatures above 200.degree. F. The double-wick concept for measuring wet-bulb temperatures was used in modified form to develop the instant energy measuring device.
Numerous forms of psychrometer are known in the prior art. The most relevant prior art of which the applicants are aware may be summarized as follows.
U.S. Pat. No. 1,601,243 to Irwin describes an early form of psychrometer in which the incoming air is heated by means of an electric heater. The psychrometer is intended for use at substantially ambient temperatures, the purpose of the heating being to prevent the formation of ice around the wet bulb.
U.S. Pat. No. 2,107,077 to Magner describes a psychrometer in which the wet bulb has a wick surrounded by a sock so that the gas the humidity of which is being measured passes over the sock and keeps the water supplied to the wet bulb at substantially the same temperature as the gas stream. This form of psychrometer is, however, capable of operating only at relatively low temperatures.
U.S. Pat. No. 2,623,391 to Malecki describes a psychrometer in which the wet bulb is surrounded by a mass of porous material to improve thermal conductivity to the wet bulb and thus to secure better sensitivity and quicker response to vapor concentration changes.
U.S. Pat. No. 3,196,683 to Gross describes a psychrometer in which, to prevent excessive evaporation of water from the wick and consequent fouling thereof at the point where the wick is exposed to the air flow, the wick is enclosed within a tube which is split so as to allow only an extremely small area of contact between the wick and the air flow.
U.S. Pat. No. 3,459,034 to Kawaguchi describes a psychrometer for measuring the moisture content of a gas at high temperatures in which a sample of gas is admitted to a measuring chamber and a wet bulb within the chamber is supplied with water from a reservoir which is kept cooled below the boiling point of water.
U.S. Pat. No. 3,515,001 to Greenspan, et al. describes a psychrometer for measuring the humidity of a stream of gas containing vapor of a condensible liquid in which a sample stream of the vapor/gas mixture is flowed over the wick and a heat exchanger, while a stream of liquid corresponding to the condensed vapor of the vapor/gas mixture is counterflowed through the heat exchanger to the wick.
U.S. Pat. No. 3,603,135 to Kawaguchi describes a high-temperature psychrometer in which the wet bulb is detachably inserted into a sleeve made of a temperature-resistant capillary material, the base portion of this sleeve being exposed to the passage of water in a predetermined amount and at a predetermined pressure so that the sleeve is maintained in a reproducibly wet condition.
U.S. Pat. No. 3,886,797 to Bauer describes an electrical-resistance psychrometer provided with means for uniform supply of an evaporating liquid from a supply tank to an evaporator body made of a porous material which surrounds the "wet" resistor, this wet resistor having an appreciable electric current passed therethrough so as to increase its temperature.
U.S. Pat. No. 4,129,250 to Chaikin, et al. describes a psychrometer intended for measuring the humidity of exhaust air from industrial driers (and thus capable of operating at moderately high temperatures) in which a wet bulb thermocouple is physically dipped into a bowl of water under the cotrol of a timer system to control the amount of exhause air discharhed until the wet bulb thermocouple is equilibrated with the exhause air sampled after dipping.
U.S. Pat. No. 4,222,261 issued Sept. 16, 1980 to Leblanc, et al. describes a high-temperature psychrometer in which the wet bulb is surrounded by a sheath of porous material, which is in turn surrounded by a screen to prevent thermal radiation evaporating too much liquid from the sheath. A dosing pump is provided for injecting predetermined amounts of volatile liquid at regular intervals into the interior of the sheath and onto the screens to ensure sufficient moistening of the sheath and the screens.
Zagorzycki, "Automatic Control of Conveyor Driers", Chemical Engineering Practice 75(4), 50 (1979) discusses the drying of food products and the limitations of prior art psychrometers.